. Cell heredity. Cytogenetics. o- (b). (c) FIGURE Diagram of Belting's hypothesis of chromosome duplication, plus the added effects of sister-strand crossing over. (o). The linked series of open dots indicate one chromosome (double helix) and the black dots indicate a paired homologous chromosome. According to Belling, the dots represented genes,- on a more modern view, consider the dots as nucleotide pairs. The parental chromosomes are twisted about each other, and new strands form in association partly with one homologue and partly with the other. (b). Consequences of this model: only t
. Cell heredity. Cytogenetics. o- (b). (c) FIGURE Diagram of Belting's hypothesis of chromosome duplication, plus the added effects of sister-strand crossing over. (o). The linked series of open dots indicate one chromosome (double helix) and the black dots indicate a paired homologous chromosome. According to Belling, the dots represented genes,- on a more modern view, consider the dots as nucleotide pairs. The parental chromosomes are twisted about each other, and new strands form in association partly with one homologue and partly with the other. (b). Consequences of this model: only two of the four chromatids can be recombi- nant. To obtain recombination among all four strands an additional step of sister- strand crossing over is postulated. (c). An example of the effects of sister-strand crossing over imposed upon Belling's model. (o and b after Swanson, 1957, Cytology and Cytogenetics, Englewood Cliffs, N. J., Prentice-Hall, p. 288). markers in a functionally unchanged condition. In fact, the reliability of these results provided the great predictive strength of classical genet- ics. Subsequently, using half-tetrads in Drosophila, it was shown that not only did single crossovers occur with a fairly constant frequency between any two genes but also that two-, three- and four-strand double crossovers occurred with the frequencies predicted on the basis of randomness of exchange between any two of the four chromatids. The latter result was referred to in the literature as the "absence of chromatid interference" in crossing over. Unfortunately, the results obtained are not really conclusive because the interpretation involves the assumption that sister-strand crossing over does not occur. In 1931, Belling proposed that crossing over occurred at the time of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly res
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